We have to supply power and transportation to an eventual population of 10 billion people who deserve decent lives, and we have to do it while limiting the emissions that threaten our collective future.

Yet we have already poured so much carbon dioxide, the main greenhouse gas, into the atmosphere that huge, threatening changes to the world’s climate appear to be inevitable. And instead of slowing down, emissions are speeding up as billions of once-destitute people claw their way out of poverty, powered by fossil fuels.

Many environmentalists believe that wind and solar power can be scaled to meet the rising demand, especially if coupled with aggressive efforts to cut waste. But a lot of energy analysts have crunched the numbers and concluded that today’s renewables, important as they are, cannot get us even halfway there.

“We need energy miracles,” Mr. Gates said in a speech three years ago introducing his approach, embodied in a company called TerraPower.

A variety of new technologies might help. Bright young folks in American universities are working on better ways to store electricity, which could solve many of the problems associated with renewable power. Work has even begun on futuristic technologies that might cheaply pull carbon dioxide out of the air.

But because of the pressing need for thousands of large generating stations that emit no carbon dioxide while providing electricity day and night, many technologists keep returning to potential improvements in nuclear power.